Ear

Ear

The human ear is the organ responsible for hearing and balance. The ear consists of three parts: the outer, middle, and inner ears.

Outer ear

The outer ear collects external sounds and funnels them through the auditory system. The outer ear is composed of three parts, the pinna (or auricle), the auditory canal, and the eardrum (tympanic membrane).

What are commonly called ears—the two flaplike structures on either side of the head—are actually the pinnas of the outer ear. Pinnas are skin-covered cartilage, not bone, and are therefore flexible.

The auditory canal is a passageway that begins at the ear and extends inward and slightly upwards. In the adult human it is lined with skin and hairs and is approximately 1 inch (2.5 centimeters) long. The outer one-third of the canal is lined with wax-producing cells and fine hairs. The purpose of the ear wax and hairs is to protect the eardrum (which lies at the end of the canal) by trapping dirt and foreign bodies and keeping the canal moist.

The eardrum is a thin, concave membrane stretched across the inner end of the auditory canal much like the skin covering the top of a drum. The eardrum marks the border between the outer ear and middle ear. In the adult human, the eardrum has a total area of approximately 0.1 square inch (0.6 square centimeter). The middle point of the eardrum—called the umbo—is attached to the stirrup, the first of three bones contained within the middle ear.

Middle ear

The middle ear transmits sound from the outer ear to the inner ear. The middle ear consists of an oval, air-filled space approximately 0.1 cubic inch (2 cubic centimeters) in volume. Contained in this space are three tiny bones called ossicles (pronounced OS-si-kuls). Because of their shapes, the three ossicles are known as the hammer (malleus), the anvil (incus), and the stirrup (stapes).

Words to Know

Auditory canal: Tunnel or passageway that begins at the external ear and extends inward toward the eardrum.

Cochlea: Snail-like structure in the inner ear that contains the anatomical structures responsible for hearing.

Eardrum: Also known as the tympanic membrane, a thin membrane located at the end of the auditory canal separating the outer ear from the middle ear.

Eustachian tube: A passageway leading from the middle ear to the throat.

Organ of Corti: Structure located in the cochlea that is the chief part of the ear through which sound is perceived.

Ossicles: Three tiny, connected bones located in the middle ear.

Otitis media: Ear infection common in children in which the middle ear space fills with fluid.

Otosclerosis: Hereditary disease that causes the ossicles to stiffen due to a build up of calcium.

Pinna: Also called auricle or external ear, the flaplike organ on either side of the head.

Vestibular system: System within the body that is responsible for balance and equilibrium.

Connecting the middle ear to the throat is the eustachian tube (pronounced you-STAY-she-an). This tube is normally closed, opening only as a result of muscle movement during yawning, sneezing, or swallowing. The eustachian tube causes air pressure in the middle ear to match the air pressure in the outer ear. The most noticeable example of eustachian tube function occurs when there is a quick change in altitude, such as when a plane takes off. Prior to takeoff, the pressure in the outer ear is equal to the pressure in the middle ear. When the plane gains altitude, the air pressure in the outer ear decreases, while the pressure in the middle ear remains the same, causing the ear to feel "plugged." In response to this the ear may "pop." The popping sensation is actually the quick opening and closing of the eustachian tube, and the equalization of pressure between the outer and middle ear.

Inner ear

The inner ear is responsible for interpreting and transmitting sound and balance sensations to the brain. The inner ear is small (about the size of a pea) and complex in shape. With its series of winding interconnected chambers, it has been called a labyrinth. The main components of the inner ear are the vestibule, semicircular canals, and the cochlea (pronounced COCK-lee-a).

The vestibule, a round open space, is the central structure within the inner ear. The vestibule contains two membranous sacs—the utriculus (pronounced you-TRIK-yuh-les) and the sacculus (pronounced SAC-yuhles). These sacs, lined with tiny hairs and attached to nerve fibers, function as a person's chief organs of balance.

Attached to the vestibule are three loop-shaped, fluid-filled tubes called the semicircular canals. These canals, arranged perpendicular to each other, are a key part of the vestibular system. Two of the canals help the body maintain balance when it is moving vertically, such as in falling and jumping. The third maintains horizontal balance, as when the head or body rotates.

The cochlea is the organ of hearing. The cochlea consists of a bony, snail-like shell that contains three separate fluid-filled ducts or canals. The middle canal contains the basilar membrane, which holds the organ of Corti, named after Italian anatomist Alfonso Giacomo Gaspare Corti (1822–1876) who discovered it. The organ contains some 20,000 hair cells connected at their base to the auditory nerve. The organ is the site where sound waves are converted into nerve impulses, which are then sent to the brain along the auditory nerve.

Hearing

Sound vibrations travel through air, water, or solids in the form of sound waves. These waves are captured by the pinna of the outer ear and transmitted through the auditory canal to the eardrum.

The eardrum vibrates in response to the pressure of the sound waves. The initial vibration causes the eardrum to be pushed inward by an amount equal to the intensity of the sound, so that loud sounds push the eardrum inward more than soft sounds. Once the eardrum is pushed inward, the pressure within the middle ear causes the eardrum to be pulled outward, setting up a back-and-forth motion.

The movement of the eardrum sets all three ossicles in motion. The vibrating pressure of the stirrup (last ossicle) on the small opening leading to the inner ear sets the fluid in the cochlea in motion. The fluid motion causes a corresponding, but not equal, wavelike motion of the basilar membrane.

When the basilar membrane moves, it causes the small hairs on the top of the hair cells of the Corti to bend. The bending of the hair cells causes chemical actions within the cells themselves, creating electrical impulses in the nerve fibers attached to the bottom of the hair cells. The nerve impulses travel up the auditory nerve to the brain. Loud sounds cause a large number of hair cells to be moved and many nerve impulses to be transmitted to the brain.

Hearing disorders

A problem in any part of the ear may cause a hearing disorder or hearing loss. In general, hearing loss may be caused by a birth defect, an injury, or a disease.

Birth defects may include missing pinnas, low-set pinnas, abnormalities in the size and shape of the pinnas, or a narrowing or complete closure of the auditory canal. These conditions may be corrected by surgery. Other birth defects that can affect hearing include premature birth, low birth weight, and illnesses suffered by the mother during pregnancy (such as measles). These defects damage the inner ear, specifically the cochlea. Medical treatment for this type of hearing loss is very rare. However, many individuals gain some benefit by wearing hearing aids.

Injury to the eardrum is common. A perforated or torn eardrum may be caused by a buildup of fluid in the middle ear, a direct puncture, explosion, or blast. When the normally taut eardrum is perforated, it becomes slack and does not vibrate properly. In some cases, the eardrum will heal itself without treatment. In more serious cases, surgical treatment may be necessary.

Ossicles are very susceptible to trauma. Injured ossicles may become unattached, broken, or excessively stiff. Once again, surgical treatment may correct the disorder and restore hearing.

There are a variety of diseases that can affect the ear, causing a hearing loss. Otitis media, a common condition in children, refers to an ear infection within the middle ear space. When this normally air-filled space is filled with fluid, the movement of the bones is affected and sound cannot be transmitted easily. The typical treatment for otitis media is medication (antibiotics and decongestants). Otosclerosis is a disease that causes the ossicles to stiffen due to a build-up of calcium. It is a hereditary disorder (inherited through family), develops in early adulthood, and is more common in women than men. Treatment may include surgery or the use of a hearing aid.

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ear

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

ear, organ of hearing and equilibrium. The human ear consists of outer, middle, and inner parts. The outer ear is the visible portion; it includes the skin-covered flap of cartilage known as the auricle, or pinna, and the opening (auditory canal) leading to the eardrum (tympanic membrane).

The middle ear, separated from the outer ear by the eardrum, contains three small bones, or ossicles. Because of their shapes, these bones are known as the hammer (malleus), anvil (incus), and stirrup (stapes). Air reaches the middle ear through the Eustachian tube, or auditory tube, which connects it to the throat.

The inner ear, or labyrinth, contains the cochlea, which houses the sound-analyzing cells of the ear, and the vestibule, which houses the organs of equilibrium. The cochlea is a coiled, fluid-filled tube divided into the three canals: the vestibular, tympanic, and cochlear canals. The basilar membrane forms a partition between the cochlear canal and the tympanic canal and houses the organ of Corti. Anchored in the Corti structure are some 20,000 hair cells, with filaments varying in length in a manner somewhat analogous to harp strings. These are the sensory hearing cells, connected at their base with the auditory nerve.

The Hearing Process

In the course of hearing, sound waves enter the auditory canal and strike the eardrum, causing it to vibrate. The sound waves are concentrated by passing from a relatively large area (the eardrum) through the ossicles to a relatively small opening leading to the inner ear. Here the stirrup vibrates, setting in motion the fluid of the cochlea. The alternating changes of pressure agitate the basilar membrane on which the organ of Corti rests, moving the hair cells. This movement stimulates the sensory hair cells to send impulses along the auditory nerve to the brain.

It is not known how the brain distinguishes high-pitched from low-pitched sounds. One theory proposes that the sensation of pitch is dependent on which area of the basilar membrane is made to vibrate. How the brain distinguishes between loud and soft sounds is also not understood, though some scientists believe that loudness is determined by the intensity of vibration of the basilar membrane.

In a small portion of normal hearing, sound waves are transmitted directly to the inner ear by causing the bones of the skull to vibrate, i.e., the auditory canal and the middle ear are bypassed. This kind of hearing, called bone conduction, is utilized in compensating for certain kinds of deafness (see deafness; hearing aid), and plays a role in the hearing of extremely loud sounds.

Balance and Orientation

In addition to the structures used for hearing, the inner ear contains the semicircular canals and the utriculus and sacculus, the chief organs of balance and orientation. There are three fluid-filled semicircular canals: two determine vertical body movement such as falling or jumping, while the third determines horizontal movements like rotation. Each canal contains an area at its base, called the ampulla, that houses sensory hair cells. The hair cells project into a thick, gelatinous mass. When the head is moved, the canals move also, but the thick fluid lags behind, and the hair cells are bent by being driven through the relatively stationary fluid. As in the cochlea, the sensory hair cells stimulate nerve impulses to the brain. The sensory hair cells of the saclike utriculus and sacculus project into a gelatinous material that contains lime crystals. When the head is tilted in various positions, the gelatin and crystals exert varying pressure on the sensory cells, which in turn send varying patterns of stimulation to the brain. The utriculus sends indications of the position of the head to the brain and detects stopping and starting. The utriculus and sacculus also help control blood flow to the brain.

Disorders of the Ear

One of the most common ear diseases is known as otitis media, a middle ear disorder. Most common among young children, otitis media probably results from Eustachian tubes that are shorter and more horizontal than in adults, allowing infection to spread and preventing fluids in the middle ear from draining. It can bring about permanent hearing loss, although modern medication is generally able to clear up the disease. Serious cases may require drainage of collected fluids through an incision in the eardrum or insertion of a tiny drainage tube. Other ear diseases include otosclerosis, involving excessive bone growth in the middle ear, and presbycusis, the progressive decay of the inner ear's hearing nerve.

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ear

ear1
/ i(ə)r/
•
n.
the organ of hearing and balance in humans and other vertebrates, esp. the external part of this. ∎
an organ sensitive to sound in other animals.
∎
an ability to recognize, appreciate, and reproduce sounds, esp. music or language:
an ear for melody. ∎
used to refer to a person's willingness to listen and pay attention to something:
she offers a sympathetic ear to worried pet owners. ∎
an ear-shaped thing, esp. the handle of a jug.
PHRASES:be all ears inf.
be listening eagerly and attentively.one's ears are burning
one is subconsciously aware of being talked about or criticized.have something coming out of one's ears inf.
have a substantial or excessive amount of something:
that man's got money coming out of his ears.have someone's ear
have access to and influence with someone:
he claimed to have the prime minister's ear.have
(or keep) an ear to the ground
be well informed about events and trends.in
one ear and out the other
heard but disregarded or quickly forgotten:
whatever he tells me seems to go in one ear and out the other.up to one's ears in inf.
very busy with or deeply involved in:
I'm up to my ears in work here.DERIVATIVES:eared
adj. [in comb.]
long-eared.ear·less
adj.ear2 •
n.
the seed-bearing head or spike of a cereal plant. ∎
a head of corn.

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ear

ear (eer) n. the sense organ concerned with hearing and balance. Sound waves, transmitted from the outside into the external auditory meatus, cause the eardrum (tympanic membrane) to vibrate. The small bones (ossicles) of the middle ear – the malleus, incus, and stapes – transmit the sound vibrations to the fenestra ovalis, which leads to the inner ear (see labyrinth). Inside the cochlea the sound vibrations are converted into nerve impulses. Pressure within the ear is released through the Eustachian tube. The semicircular canals, saccule, and utricle – also in the inner ear – are all concerned with balance.

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ear

ear Organ of hearing and balance. It converts sound waves to nerve impulses which are carried to the brain. In most mammals, it consists of the outer, middle and inner ear. The outer ear carries sound to the eardrum. The middle ear is air-filled, and has three tiny bones (ossicles) that pass on and amplify sound vibrations to the fluid-filled inner ear. The inner ear contains the cochlea and semi-circular canals. Vibrations stimulate tiny hairs in these organs which cause impulses to be sent via the auditory nerve to the brain. The inner ear also contains semi-circular canals that maintain orientation and balance.

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ear

ear The sense organ in vertebrates that is specialized for the detection of sound and the maintenance of balance. It can be divided into the outer ear and middle ear, which collect and transmit sound waves, and the inner ear, which contains the organs of balance and (except in fish) hearing (see illustration). The term ear is often used for the pinna of the mammalian outer ear.

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